This invention relates generally to cranks for a pedal crank drive for bicycles or the like, and especially to cranks having variable lengths.
Bicycles and other pedal driven vehicles typically include a drive assembly which includes a bearing tube mounted to the bicycle frame, a spindle mounted for rotary movement within the bearing tube, a pair a crank arms mounted to the spindle, and a pedal mounted to the end of each crank arm. A first sprocket is coupled to the crank arm which has an endless loop chain mounted thereon which is also coupled to a second sprocket coupled to the rear wheel of the bicycle. The bicycle is manually driven by the rider exerting pressure upon the pedals which cause rotation of the cranks arms, spindle and first sprocket. The movement of the first sprocket causes movement of the chain and second sprocket, which in turn, causes movement of the rear wheel.
It is well known that the length of the crank arm is one factor which controls the amount of force that can be transferred to the first sprocket. Generally, the longer the crank arm is the greater the amount of torque it can transfer from the rider to the first sprocket. However, the anatomy of a human limits the length of a crank arm, as the distance between opposing pedals must allow the rider to reach both pedals at all times during the complete cycling of the pedal. A human""s anatomy allows a greater range of leg motion in the forward and rearward, or horizontal, direction than it does in the up and down, or vertical, direction. Therefore, the length of the bicycle crank is limited by the range of leg movement in the vertical direction. However, the greatest leverage is produced by the leg in the downward stroke portion of the pedally cycle. As such, it would be advantageous to have a crank arm extended during this portion of the pedaling cycle.
To this end, others have designed variable length crank arms to enable the crank arm to have a longer length during select portions of the pedaling cycle. U.S. Pat. No. 4,807,491 shows a crank assembly having a telescoping crank arm to vary the length of the arm. The rider manually slides the crank arm to its extended and retracted positions during the pedaling cycle. While this aids in increasing the length of the crank the rider must concentrate on the sliding movement at all times, thereby distracting the rider from other aspects which need attention.
Variable length crank arms have also been designed wherein the crank is mounted to a position offset from the spindle, as shown in U.S. Pat. No. 6,152,471. The offset position of the crank varies the length of the crank arm according to the position of the crank arm within the pedaling cycle, i.e., the crank arm is at its longest length during the downward stroke of the cycle. This type of device however is complicated to manufacture and awkward to control.
Accordingly, it is seen that a need remains for a crank arm assembly that provides an easy to operate variable length crank arm assembly which is also simple to manufacture. It is the provision of such that the present invention is primarily directed.
In a preferred form of the invention a variable length crank arm assembly for a bicycle having a frame and a rotatable spindle comprises a crank arm coupled to the spindle for rotation about a rotational axis of the spindle. The crank arm has a longitudinal axis, a proximal stop adjacent the spindle and a distal stop distally from said spindle. The crank arm assembly also has a slide mounted upon the crank arm for slideable movement thereon between a proximal location proximal the spindle and a distal location distal the spindle, a pedal rotatably mounted to the slide, a first spring mounted between the slide and the proximal stop to bias the slide toward the distal stop along the crank arm longitudinal axis, and a second spring mounted between the slide and the distal stop to bias the slide toward the proximal stop along the crank arm longitudinal axis. With this construction, an operator may force the slide along the crank arm against the biasing force of the second spring to elongate the stroke of the variable length crank arm and then allow the biasing force of the second spring to return the slide toward the proximal stop, and whereby the operator may force the slide along the crank arm against the biasing force of the first spring to shorten the stroke of the variable length crank arm and then allow the biasing force of the first spring to return the slide toward the distal stop.